The Astronic A1671 is an typical active reciprocal design, (not passive) with all discrete amp & transformer in/out , much like our Quad-Eight & Electrodyne graphics. Note the overall massive size of the inductors (largest is about 36mm dia.) that increase in size proportionally with decrease in frequency band handled.
The eq circuit is as expected, a set of classic LC inductor nets, each connected to the wiper of its own boost/cut pot, & hung across the differential input pins of the eq amp.
I do not recall if this design uses two eq amps (QEE EQ-712 film graphic), or only a single amp stage like the lever graphic in our Electrodyne 712L channel strips and its direct descendants from Sphere (same talented design team, just a different company "flag").
The attached images are a unit I found lying in the corner of a Movie prop house I used to manage in LA, and had no front panel or identifying marks, but for some reason it said "grab me, I can do cool things to audio".
It sat on a shelf in the Orphan Audio shop for about 15 years, until I found an internet thread with internal pix that finally identified it as an Astronic, and I then set about the detailed process of completely rebuilding the fragile and abused phenolic gain sliders, graphic design and fab of a proper replica engraved front panel, digging the right period accurate switches, knobs and lamps out of the spares bins, full recap, amplifier repairs and conversion to 110vac operation.
I did take detailed notes and pictures and have a mostly complete (and barely readable) schematic penciled out. If I can get my arms pulled out of the long line of electronic "alligators" waiting to occupy my tech benches, I will complete it in CAD so its at least a usable reference.
As to sonics,.. Well all I can say is,.... it sounds HUGE !!!
If this project is something too tantalizing for the GDIY community to wait ,.... I am happy to forward my notes and contact info to any willing tech who will finish the schematic process. No secrets to be found here, just great engineering, design and component selection.
Ken Hirsch
Orphan Audio
Quad-Eight Electronics
Electrodyne Audio
